Allow crawl space threshold to be set on CLI, report space avail

[machawk1]

Closes #61

@CorentinB Please note, I'm not proficient in Go, so feedback is welcomed and edit at will.

[CorentinB]

Thanks for your contribution @machawk1, to be noted: WARC writing is async. The WARC writing queue is displayed when using Zeno with --live-stats, one edge case that is not handled by my code currently is when disk space is too low so the crawl pause, but at the same time there is enough in the WARC writing queue that the remaining space can be eaten up entirely.

Because the WARC writing isn't paused by this pause mechanism here. I mean the WARC writing that was already in the WARC writing queue. I hope it's clear.

Not saying you have to do anything about it here, and solving this would require some additional synchronization between Zeno and the underlying warc library, but I thought it was worth mentioning. :)

[CorentinB]

Thanks! Really sorry for the delay here..